In semiconductor manufacture lithography, in which a reticle pattern is projected and transferred onto a wafer by exposing the pattern using exposure light, an apparatus for inspecting the exposure and defocus amount is used.
The conventional flow of lithography will be described.
A resist pattern is formed by applying a resist, serving as a photosensitizer to a substrate, such as a semiconductor wafer, projecting a mask pattern on the resist by exposure using an exposure apparatus, and then, developing the resist.
The dimensions of the formed resist pattern are checked by a scanning electron microscope (measuring SEM or CD-SEM) with a measuring function. The conventional processing contents of the measuring SEM include, e.g., acquiring an electron beam image of an area containing parts with strict dimensional accuracy (step 1), measuring the dimensions (step 2), determining whether the dimensions meet the standards (step 3), and, if not, changing the exposure of the exposure apparatus (step 4, the exposure correction amount is ΔE). For, e.g., a positive resist, if the resist dimensions are too large, the exposure is increased. If the resist width is too small, the exposure is decreased.
The relationship between a resist pattern and a film pattern after etching will be described next.
The shape of a resist pattern and the shape of a film pattern have a predetermined relationship if the etching conditions are the same. To obtain a film pattern having a predetermined shape, the resist pattern must have a predetermined shape, too. In, e.g., starting a new process, a wafer called an FEM (Focus Exposure Matrix) is prepared by exposing a pattern while changing the focus and exposure in each shot (one exposure cycle). A focus and exposure to obtain a predetermined resist pattern shape are found by measuring the dimensions of the resist pattern in each shot, and cutting the wafer to check its sectional shape. That is, a so-called “condition determination” is performed.
With this operation, the exposure (E0) and focus value (F0) to widen the margin are determined. Product wafers are exposed on the basis of these conditions. However, it may be impossible sometimes to obtain a resist pattern with a predetermined shape under the conditions (E0, F0) determined by the “condition determination,” because of various process variations (e.g., a change in resist sensitivity, a variation in thickness of an antireflection film under the resist, or a drift of various kinds of sensors of the exposure apparatus). This is detected in dimension measurement (step 2). In the conventional technique, the change in resist shape caused by process variations is compensated for by correcting the exposure.
KLA-Tencor in the United States announced, on Jun. 24, 2003, an “MPX” that enables in-line focus/exposure monitoring as a new option of an overlay measuring apparatus in their “Archer” series. The “MPX” can monitor the focus and exposure by analyzing a unique dual tone line end shortening (LES) target and accurately separating the focus and the exposure. On the basis of the data, management of the defocus and exposure of the exposure apparatus, grasping the apparatus variation, and specifying the cause can be done quickly. KLA-Tencor states that users can suppress any decrease in yield associated with focus, and save the cost of millions of dollars a year by using that option. Techniques related to such a measuring apparatus are disclosed in U.S. Pat. No. 5,629,772, U.S. Pat. No. 5,757,507, U.S. Pat. No. 5,790,254, U.S. Pat. No. 6,137,578, and U.S. Pat. No. 6,577,406.
In the prior art, to detect and to cope with process variations, dimensional values, such as a line width are checked by using measuring SEM. If the dimensional values do not meet the standards, the exposure is corrected. However, this method has the problem of focal depth degradation described below.
When the exposure changes, the line width changes. On the other hand, the line width rarely changes, even when the focus changes. However, when the focus changes, the sectional shape of the resist changes, although the line width does not change. As described above, the change in sectional shape influences the shape of the film pattern after etching. For this reason, since the prior art is incapable of detecting a focus variation, poor film pattern shapes may be produced in large quantities after etching.
As described above, since displacement caused by defocus cannot be corrected by correcting only the exposure, the resist can have no normal sectional shape. In addition, since exposure is not executed at the center of the depth of focus, the depth may be insufficient, and poor film pattern shapes may be produced in large quantities after etching.
In “MPX”, the focus amount and exposure are estimated by analyzing a dual tone line end shortening target (to be referred to as a mark, hereinafter). The mark includes a hollow grating mark and a solid grating mark, as shown in FIG. 1. Conventionally, the focus amount and exposure are estimated by measuring intervals CD1 and CD2 shown in FIG. 1. The defocus amount is estimated by using the behavior of the interval CD1 or CD2, which changes with respect to the focus amount, as shown in FIG. 2.
FIG. 2 shows the relationship between the interval CD1 or CD2 and the focus amount at an exposure E=E0. The interval CD1 or CD2 is plotted along the ordinate, and the focus amount is plotted along the abscissa. Z0 is the optimum focus amount. The interval CD1 or CD2 is minimized when the focus amount is Z0. However, the interval CD1 or CD2 changes as an even function with respect to the defocus amount. For this reason, the interval is usable in estimating the defocus amount, but not usable in estimating the defocus direction (i.e., whether the defocus occurs in the positive direction or the negative direction).